SULFIDE CORE SAMPLE FROM THE DEEP-SEA ARTIFICIAL HYDROTHERMAL VENT AT THE IHEYA-NORTH KNOLL, OKINAWA TROUGH: NATURAL ANALOGUE OF SUB-SEAFLOOR MINERALIZATION ON A SEAFLOOR

In 2016, CK16-01 Cruise by D/V Chikyu was conducted at the Okinawa Trough. In this cruise, we planned to remove a scale within a steel pipe of the artificial hydrothermal vent (Hole C0014G) and to install a temporal flowmeter to measure P/T and flow rate before/during coring operations. We developed various hydraulic tools with an independent hydraulic power unit which supplies hydraulic power to various tools independent of the ROV hydraulic system. At first, a short drill with a 5-inch stroke was used. The drill did not penetrate the scale, but 3 cm long sulfide core was obtained. Then, a long drill with a 16-inch stroke was used. The long drill penetrated the scale with vigorous hydrothermal effluent (200 L/min) and 20 cm long sulfide core was obtained. Here, we report petrological and geochemical signatures of these sulfide cores. The sulfide core is massive and exhibits black color. Several reddish-brown color bands (1 mm width) are intercalated, which show a concentric ring structure. Under the microscope, sphalerite is dominated with minor galena and chalcopyrite. In XRD analyses, sphalerite, galena and chalcopyrite peaks were only detected. From outer to inner sides, the sulfide core has a zonal structure of (1) sphalerite dominant part whose crystal is oriented to center, (2) dendritic galena dominant part, (3) spherical/colloform chalcopyrite and sphalerite dominant part and (4) dendritic galena dominant part. Barite crystals were observed only from the uppermost/center part. These sulfide cores have average contents of 57.4% Zn, 1.14% Pb, 0.31% Cu, 0.31% Cd, 24.3 ppm Ag and 0.19 ppm Au. Fe and Cu contents decrease in ascending order, whereas Zn, Sb and Ba contents increase. The reddish-brown part is rich in chalcopyrite and galena. Scale removal operations were previously tried six times, inducing the zonal structure of chalcopyrite-rich part formed under a relatively high-temperature condition and sphalerite-rich part formed during a waning stage of hydrothermalism. Sulfide core was formed by conductive cooling without seawater mixing, which is consistent with the limited occurrence of sulfate minerals. Thus, within the steel pipe, sulfide mineralization without seawater mixing was reproduced on a seafloor and deep-sea artificial hydrothermal vent is “a peeping hole of sub-seafloor sulfide mineralization”.